Seed-warming mats help gardeners grow seeds indoors during the cold winter months when outdoor gardening is not possible. These mats maintain a constant temperature controlled by a thermostat. Rather than buying a commercially available seed mat from a store, consider using a waterbed heater instead. A waterbed heater functions just as well as a store-bought seed-warming mat, as long as it contains a thermostat.
Examine the water bed heater to locate the temperature sensor on the thermostat. The temperature sensor looks like a small metal projection protruding from the thermostat. Attach the sensor to the mat using duct tape. The sensor regulates the temperature so the heating element shuts off when the mat achieves the desired temperature.
Lay the water bed heater out on a flat surface where you will store the seed tray. Plug the heater into an electrical outlet.
Turn the thermostat on the water bed heater to the lowest setting.
Place four pieces of wood or bricks at each corner of the heater mat. It does not matter how large the pieces of wood or bricks are, as long as they prevent the tray from resting on the heating mat itself.
Place the seed tray on top of the bricks or wood and fill each section with potting soil.
Insert a seed into the center of each section and push it a quarter-inch into the soil.
Water the seed tray with one cup of water distributed evenly throughout the tray sections.